(Update: Zhai Xiaobing’s name was misspelled in an earlier version of this story.)

As jokes go, it was hardly funny. Ahead of China’s 18th Communist Party Congress, which saw the top leadership positions transfer for the first time in a decade, Beijing investment banker Zhai Xiaobing tweeted a spoiler of an imaginary film he called Final Destination 6. A great hall will collapse, and out of the 2,000 people inside, just seven will live, but the survivors will begin to mysteriously die off, he tweeted on Nov. 5 under his handle @stariver. “Is it a game of God, or the wrath of Death? How will 18, the mysterious number, unlock the gate of Hell?” he wrote. “Premieres globally on November the 8th.”

The tweet combined the plots of the Final Destination teen horror films with the Party Congress, which started on Nov. 8 and culminated with the announcement of a new seven-member Politburo Standing Committee, China’s top governing body. Zhai’s friends recognized his message as a parody. Beijing police, who were in the midst of an extensive security crackdown ahead of the Party Congress, didn’t see the humor. The local authorities had forced prominent dissidents out of town, restricted sales of knives, Ping-Pong balls (on which subversive messages could be written) and remote-control model aircraft and ordered taxi drivers to remove window handles. So jokes about the Communist Party’s leadership dying in a disaster were out. Zhai, 36, who studied ancient Chinese literature at Peking University, was detained Nov. 7 by police in Miyun, a Beijing suburb, on suspicion of spreading false terrorist information, according to friends.

Word of Zhai’s detention, which only emerged after the Party Congress ended on Nov. 14, provoked indignation among his friends and Twitter followers. “Anyone who has seen the Final Destination movies will understand he was just cracking a joke, not fabricating untruths to endanger state security,” said a longtime friend of Zhai’s who asked not to be named because of the risk of arrest. “If the police send him to a labor camp or even sentence him to prison because of something he posted on Twitter, that’s just totally unacceptable.” As of Nov. 21, more than 500 people had signed an online petition calling for his release.

Zhai’s case highlights the push and pull of free speech in China. Domestic Chinese microblogs like the popular Sina Weibo service, which recently announced it had more than 400 million registered accounts, are censored, with searches for sensitive words blocked and edgy posts removed. But users can still write highly critical commentary and firsthand accounts of breaking news, like recent environmental protests. Such material is often deleted, but time lags mean that many users can see sensitive posts before they disappear. On Twitter, which is blocked in China but accessible by using virtual private networks, Chinese commentary flows more freely. Many Chinese liberals and activists jump through the technical hoops to post on Twitter without fear of having their comments deleted.

The result is an unpredictable environment. Chinese are vastly freer to speak their opinions now than they were during the Mao era, and the most common response for online criticism of the government is deletion rather than detention. Even complaints can find a place within the restricted space of the Chinese Internet. A recent study by Harvard researchers, who collected data on millions of social-media posts in China, found that the chief priority of the country’s censorship regime was not to block negative views of the leadership but to restrict information that might spur a collective response.

Of course, online criticism of Chinese officialdom doesn’t necessarily go ignored. A U.S. State Department cable released by WikiLeaks in 2010 said that China initiated an investigation into Google after a Politburo Standing Committee member, later identified by the New York Times as propaganda czar Li Changchun, discovered that criticism of him in Chinese could be found after he used Google to search his own name.

In the southwestern city of Chongqing, where Bo Xilai was party boss until his dramatic downfall this spring, several people were detained and sentenced to labor camps for positing criticism of Bo and his policies. Ren Jianyu, a 25-year-old former village official, was released from a re-education-through-labor camp on Nov. 19. He served about half of a two-year sentence for posting comments that linked Bo’s campaign promoting Maoist-era songs with the Cultural Revolution, the 1966–76 period of widespread political violence in China. Ren’s attorney, Pu Zhiqiang, said no specific reason was given for canceling his client’s sentence. It undoubtedly helped that some of Bo’s most powerful opponents, including Premier Wen Jiabao, made similar criticisms of his flashy campaigns in Chongqing. With new leaders installed in Chongqing following the Party Congress, Pu says he expect the limits on speech to ease. “No one, not even Bo Xilai, can keep these coercive policies going for a long time,” he says. Still, he notes that at least 10 people remain in detention for criticism of Bo.

In a recent speech at Peking University, prominent blogger Li Chengpeng said he worried that restrictions on speech in China risked repeating disasters of earlier eras, when citizens and even officials couldn’t question bad policies. “We’re like the world’s largest army of mutes marching forward,” he said, according to a translation by Liz Carter on the blog A Big Enough Forest. “The most terrifying thing about this country isn’t poverty, or hunger, or not winning a Nobel Prize, or not realizing a high enough GDP, or not producing enough party reports, it’s that the people have lost their ability and right to speak.”

For those China-haters, have you been to a detention center in China. Even Liu Xiaobo says that the prison condition in China today is way more tolerable than ten or twenty years back. The condition, of course, varies from city to city.

There is a very funny thing in Vietnam: Facing jail for protesting China for invading Vietnam.

One extreme case of The emperors' power is unparalleled: A Vietnamese young woman (Phạm Thanh Nghiên) protesting China invasion of her country inside her house; posting her picture on the internet, got arrested by vietnam police; spent 4 years jail plus 3 years under house arrest! I wish some people can develop software/build device that make China's firewall totally useless.

I love it "Chinese are vastly freer to speak their opinions now than they were during the Mao era" Anything short of immediate execution by a firing squad would be "freer speech" then what the Chinese had under Mao.